1. Field of the Invention
The present invention relates to a radial tire for heavy duty, more particularly to an improvement of the structure for reinforcing bead sections of heavy duty radial tires.
2. Description of the Prior Art
In general, as shown in FIG. 1, a tire of this type comprises: a single ply carcass 102 arranged radially or semi-radially at a small angle of 15.degree. to 30.degree. with respect to the equatorial line of the tire; a hard rubber stock 111 and a soft rubber stock 112 each disposed above a bead core 103 between the main portion and turned up portion 102' of the carcass ply 102 and having JIS hardness greater than 80.degree. and of about 50.degree. to 65.degree., respectively; a reinforcing layer 104 composed of at least one rubberized ply containing metal cords embedded therein and disposed at the outside of the turned up portion 102'.
As another one example of the prior art, Japanese patent No. 967452 is well known to be one of excellent structure for reinforcing bead section. This patent, as shown in FIG. 2, is characterized in that a fiber cord reinforcing layers 206 composed of two or more layers having rubberized organic fiber cords and crossed with each other are arranged at the outside of the above mentioned metal cord first reinforcing layer 204 and extend radially upwardly from the bead base to the side wall region.
In this way, the radial or semi-radial tire has a single ply carcass composed of radially directed cords. As a result, the side wall of the tire is extremely soft, and its flexure in the axial direction of the tire is very large.
Therefore, the amount of deformation of the radial or semi-radial tire during one rotation is far larger than that of a bias tire. That is, the bead section of the tire is strongly forced to deform like a convex surface and overhang the rim flange under the influence of such soft side walls, contributing to their temperature increase.
It has been found by the inventors that the internal temperature of the bead section may increase up to 120.degree. C. to 170.degree. C. due to the above mentioned temperature increase and the heat transmission from the brake drum of vehicles. Thus, the bead sections suffer from dynamic fatigue and also thermal fatigue.
Furthermore, the adhesion of rubber with metallic cords or fiber cords, which is vital to maintain the tire performance, is largely detrimentally influenced by the dynamic and thermal fatigue when the internal temperature of the tire increased.
On the other hand, in these types of radial tires, the worn tire can be repeatedly used twice or thrice by recapping, and this recapping is regarded as a sales point for steel radial tires.
This recappability depends on the durability of the bead section at present. However, the above mentioned tire of the prior art do not always satisfy the requirement of such durability.
Recently, some drivers have begun to use such tires under more severe service conditions, namely, long and high-speed running under heavy load and high inflation pressure.
Accordingly, further improvement of the durability of the bead sections is strongly required.
When the tire having the bead section shown in FIG. 2 is inflated, the strain action in the bead section is illustrated as FIG. 3. That is, the carcass ply 202 turned up around the bead core 203 moves upwardly as shown by an arrow 21. The outermost end 202a of the turned-up portion 202' moves downwardly as shown by an arrow 23. As a result, the turned up portion 202' causes the reinforcing layer 204 and 206 to move downwardly as shown by an arrow 22, with rotating the bead core 203 as shown by an arrow 24.
When the adhesion among these layers is unable to endure the imposed strain, the carcass ply 202 pulls out from the bead core 203, this phenomenon will hereinafter be referred as "blow out".
This blow out phenomenon is apt to occur acceleratively by the above mentioned internal temperature raise, and the higher the temperature, the earlier it occurs.
Next, to consider the dynamic strain caused by the tire revolution under heavy load, the grounding area of the tire is most subjected to deformation, and the radially inner portion of the side wall strongly forced to deform like a convex surface and overhang the rim flange under the influence of such soft side walls. Therefore, the cord space between the adjacent steel cords is expanded by the deformation at the radially outer portion of the bead section, and in company with such movement of the carcass cord, the turned up portion 202' and the metal cord reinforcing layer 204 are forced to move toward the outerside of the axial direction of the tire, as the result, the outermost end 202a of the turned up portion 202' and the outermost end 204a of the steel cord reinforcing layer 204 are more subjected to stress concentration, which leads to the separation in the bead sections.
With respect to the above mentioned internal temperature and strain in the bead section, various investigations were made, and the following facts were found: in a structure having the fiber cord reinforcing layers 206 arranged outside the above mentioned metal cord reinforcing layer 204 (See FIG. 2) as proposed in Japanese Patent No. 967452, the bead section is made thick, and more heat is generated under heavy load, and the strain in the bead section more increased.
As a result, the cord end of the fiber cord reinforcing layer 206 is apt to be the nucleus of causing separation due to stress concentration of the cord ends of the carcass ply 202 and the metallic cord reinforcing layer 204. And sometime the organic fiber cords in the fiber cord reinforcing layers 206 are broken, depending on the angle at which the fiber cord reinforcing layer (206) is arranged. And also the cut ends are apt to become the nucleus of causing separation in the bead section, when the tire undergo large lateral deflection.